1. Field of the Invention
The present invention relates to a DC/DC converter, in particular, to a technology for reducing the power consumption of a DC/DC converter and for improving the converting efficiency.
2. Description of the Related Art
A DC/DC converter is an apparatus that converts a particular DC input voltage into another DC voltage and outputs the converted DC voltage. The DC/DC converter is used for various systems. The output voltage of the DC/DC converter should be free from fluctuation. When the DC/DC converter is installed in a portable terminal or the like, the power consumption of the DC/DC converter should be as small as possible.
FIG. 1 is a circuit diagram showing a structure of a conventional DC/DC converter. The DC/DC converter shown in FIG. 1 converts an input voltage V.sub.in into an output voltage V.sub.out. The DC/DC converter controls the power to be supplied to a load corresponding to a PFM (pulse frequency modulation) method. In the PFM system, when the load requires a high current, the pulse frequency (pulse generating frequency) is increased. When the load does not require a high current, the pulse frequency is decreased.
A pulse frequency modulator 1 receives the output signal V.sub.out as a feedback signal and outputs a pulse signal for causing a switching device 3 to be turned on with a pulse frequency corresponding to the voltage V.sub.out. A driver 2 turns on the switching device 3 for a predetermined time period whenever the driver 2 receives the pulse signal from the pulse frequency modulator 1. The switching device 3 is turned on or off corresponding to an on/off control signal received from the driver 2. The switching device 3 is composed of, for example, a P-channel type MOS transistor. When the signal level of the output signal of the driver 2 is "L", the switching device 3 is turned on.
When the switching device 3 is turned on, a current flows from the switching device 3 to a condenser C through a coil L. An electrical charge is stored in the condenser C and thereby the output voltage V.sub.out increases. Thus, when the output voltage V.sub.out decreases to a designated voltage or less, the pulse frequency modulator 1 increases the pulse frequency and thereby increases the number of pulses for turning on the switching device 3. On the other hand, when the output voltage V.sub.out increases and exceeds the designated voltage, the pulse frequency modulator 1 decreases the pulse frequency and thereby decreases the number of pulses for turning on the switching device 3. In the PFM method, the output voltage V.sub.out is kept constant.
FIGS. 2A, 2B, and 2C are schematic diagrams showing output signals of the pulse frequency modulator 1. In FIGS. 2A, 2B, and 2C, each pulse (a period in the "H" level) causes the switching device 3 to be turned on.
While the load current is small, as shown in FIG. 2A, the pulse frequency of the pulse frequency modulator 1 is relatively low. On the other hand, when the load current increases and the output voltage V.sub.out decreases to the designated voltage or less, as shown in FIG. 2B, the pulse frequency modulator 1 increases the pulse frequency. In other words, the frequency with which the switching device 3 is turned on is increased so as to increase the output voltage V.sub.out. In the PFM method, the output voltage V.sub.out is kept constant in such a manner.
When the DC/DC converter is installed in a portable computer or the like, the power consumption of the DC/DC converter should be reduced as much as possible so as to prolong the operation life of the terminal. However, the conventional DC/DC converter cannot satisfactorily reduce the power consumption.
In other words, when the DC/DC converter is installed in a portable computer or the like, the size of the DC/DC converter should be significantly reduced. In this case, the sizes of the condenser C and the coil L should be reduced. However, a frequency of an oscillator should be up to around several 100's of KHz, and as shown in FIG. 2C, a cycle of a pulse operation becomes short, the pulse frequency should be increased in the PFM method.
When the pulse frequency is increased, the amount of current per pulse decreases. Thus, to supply a predetermined amount of current to the load, the number of pulses should be increased. However, since the switching device 3 is turned on and off corresponding to each pulse, when the number of pulses increases, the number of times of the switching operation of the switching device 3 increases. When the switching device 3 is turned on or off, an energy loss results. Thus, the converting efficiency of the DC/DC converter deteriorates.
In the above-described structure, an oscillator 4 is essential (for example, disposed in the pulse frequency modulator 1). The oscillator 4 is always operating. In other words, in the PFM system, the oscillator 4 always oscillates at a predetermined frequency. A part of the pulse signal generated by the oscillator 4 is output as the output signal of the pulse frequency modulator 1. Thus, even if the load current is small and the pulse frequency is low, the oscillator 4 operates as with the case that the load current is large. Although the current consumed by the DC/DC converter should be decreased in proportion to the load current, the current consumed by the oscillator 4 is constant. Thus, even when the load current is small, the current consumed by the DC/DC converter cannot be decreased.
In addition, when the pulse frequency is varied, ripples of the output voltage V.sub.out become unstable. Thus, radiation noise takes place and the bandwidth of noise of the power system of the load connected to the DC/DC converter becomes wide. Consequently, it is difficult to remove such noise.
As described above, in the conventional DC/DC converter, the current consumption cannot be satisfactorily reduced and the bandwidth of noise is wide.